A book is a trusted and well-known credibility builder in the mind of those who are seeking out help on a specific subject matter- you being the author on that subject matter will instantly boost your credibility.
Scammers and fly-by-nights surely do not take the time to create a book on the services they are offering- your prospects will know you are "all in" on your service and the trust building will happy naturally.
Being seen as an expert and advocate in the eyes of your prospect with a book with position you as THE Authority in your niche and your competition wont stand a chance.